Wood pellet burner for existing boilers and furnaces

ABSTRACT

A pellet burner has inner and outer housings for defining a high pressure air plenum between. Angled perforations at strategic locations in the inner housing supplied pressurized primary and secondary combustion air so that, with pellets supplied to a burn pot of the inner housing and activation of a blower and an ignitor cartridge, the pellets auto-ignite and the air flow establishes a swirling flame in the burn pot that travels to a flame conduit of the inner housing where further swirling occurs in a venture section for shaping the flame into a clean, hot, symmetrical form.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates generally to the field of heating and, in particular, to a new and useful pellet burner for replacing the burner of an oil, propane, natural gas or other fueled boiler or furnace.

U.S. Pat. No. 4,312,278 discloses a chip wood furnace retrofitting system that seeks to replace the burner of an oil fired furnace and to solve the problem of ash buildup by establishing a combustion region above a grate through which the ash falls.

Also see U.S. Pat. No. 5,893,358 for a Pellet Fuel Burner for Heating and Drying Systems; U.S. Pat. No. 5,873,356 for a High Efficiency Wood Pellet Stove; and U.S. Pat. No. 5,183,028 for a Burning Apparatus with Pellet Fuel Burner.

A need remains, however, for an efficient replacement fuel burner that can use non-petroleum fuels such as wood pellets to fire an oil or other fueled furnace or boiler, and that can provide a flame that is as clean and hot as a gas or oil flame, with virtually no ash rising with the exhaust gases from the combustion chamber of the boiler or furnace.

SUMMARY OF THE INVENTION

The invention is a burner that burns wood pellets supplied from a source of wood pellets, the burner comprising an inner housing containing a lower burn pot which receives ignition heat and primary air through an ignition tube and lower perforation around the base of burn pot, to ignite pellets supplied to the burn pot from above, for producing a flame that is then steered horizontally through a conduit that includes a step down in diameter forming a venture space, the conduit containing a plurality of circumferentially spaced and axially spaced perforations that are each angled by a compound acute angle to the conduit axis and to the circumferential direction around the conduit, into the interior venture space, the compound angle being ideally 30 degrees, plus or minus 20 degrees, to the axial direction and 30 degrees, plus or minus 20 degrees, to the circumferential direction, and inclined toward an outlet of the venture space. The perforations provide secondary air into the venture space at a location that is spaced from the burn pot to swirl and feed the flame from the burn pot into a long, clean, hot, cylindrical, symmetrical and light ash containing flame that is well suited to file the boiler or furnace designed to be fueled by an oil or gas burner.

A directing cone in the base of the burn pot as well and circumferentially aimed primary air through the ignition tube and the perforations creates a cyclone of flame in the burn pot that rises and rotated from the burn pot, entraining all the ash and entered the horizontal venture space where the rotation is further enhanced by the swirling secondary air to fully oxygenate and shape the flame.

The inventors have found that the burner of the invention creates a closely controllable, clean flame which includes all the ash from the burning process that is advantageously and neatly deposited at the bottom of the boiler or furnace combustion chamber that has been retrofit to include the wood pellet burner of the invention, so that no ash leaves with the exhaust gases but all ash leaves the burner.

The pellets are supplied continuously from above the burner pot. When a thermostat calls for heat for example, because of a reduced temperature of water in the boiler, controls are activated to initiate an ignition temperature from the ignition tube as well as the feeding of some pellets to the burn pot. Primary and secondary air is also begun by activating a fan connected to an outer housing around the inner housing. Once a thermostat or other temperature sensor in the burner establishes that ignition has been achieved, the heating source in the ignition tube is switched off. The primary air flow and secondary air flow continue, however, and a regular continuous supply of pellet is initiated for maintaining the flame in the burn pot until the boiler or furnace thermostat determines that the temperature of water in the boiler or air in the furnace has reached the desired level. At that point a controlled shutdown of the burner is initiated until the next time heat is called for.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic perspective view of the burner of the invention with some of its interior visible and its clean, hot, substantially cylindrical and symmetrical flame illustrated, as well as a control box and pellet feeder for supplying wood pellet fuel to the burner;

FIG. 2 is a top sectional view of the burner;

FIG. 3 is a side sectional view of the burner with pellet feeder;

FIG. 4 is a view similar to FIG. 2 of an alternate burn pot for the burner;

FIG. 5 is a view similar to FIG. 3 showing a further alternate burn pot;

FIG. 6 is a top sectional view of a burner adaptor and selector mechanism of the invention for alternate use of the burner of the invention and the oil or other burner supplied with a boiler or furnace;

FIG. 7 is a view similar to FIG. 3 of a further schematic illustration of the invention and its principles of operation taken along line 7-7 of FIG. 8; and

FIG. 8 is a side sectional view taken along line 8-8 of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is a northern, green, environmentally friendly, climate-neutral, automatic, advanced, self-feeding and efficient wood pellet burner. A fuel pellet burning heating unit is a replacement for oil burners on existing boilers and furnaces or a stand alone system. The pellet burner is fed by means of an auger device from a main storage bin or hopper. Pellets enter through the top of the burner and fall by gravity, landing in a combustion chamber of the burner that is here called a burn pot.

The burn pot or combustion chamber is in an inner housing that is inside a pressurized outer housing that is continually fed air by means of a blower. Pellets are initially ignited by means of a known cartridge heater inside the ignition tube, that heats air to above the ignition temperature of the pellets. This ignition temperature is also call an auto-ignition temperature and is the temperature at which a material spontaneously ignites in the atmosphere it finds itself in, e.g. above about 500 degrees F. for wood pellets. One or more primary air perforations in the ignition tube receive pressurized air from the outer housing and this hot ignition air is directed into the burn pot in an off-center manner to swirl the air at the pellets in the burn pot. Once a flame is established, pellets are fed continuously into the burn pot or combustion chamber until the heating system is satisfied.

Combustion zone air is also introduced at the bottom of the burn pot through additional perforation in the side wall or walls of the burn pot. Combustion gases rise and enter a substantially horizontal flame conduit and are then drawn out of the inner housing by means of a venture shaped pipe or tube at the output end of the flame conduit. The venture tube has holes that are angled and through which secondary combustion air enters. These holes or secondary air perforations are angled in such a way that they not only inject secondary air into the combustion stream but also causes the gasses to be retained for cleaner burning and swirls the gas stream, increasing its velocity and its efficiency while also forming the flame into a symmetrical and substantially cylindrical shape.

Referring now to the drawings, in which like reference numerals are used to refer to the same or similar elements, FIGS. 1, 7 and 8 show an arrangement for burning pellets to fire a combustion chamber of a boiler or furnace, the arrangement comprising an outer housing 10 defining a first air space with an air inlet 14 for air and an outer housing outlet 16. An air blower 20 is connected to the housing 10 for supplying air to the air inlet 14 and into the first air space for pressurizing the first air space.

An inner housing 30 is mounted in the outer housing 10 with the first air space surrounding at least some of the inner housing for defining an air plenum 12 between the inner and outer housings, the inner housing 30 defining a vertically extending burn pot 32 having an upper end and a lower base 34 with a side wall 36 extending around and vertically from the base 34. The side wall 36 includes a plurality of, e.g. 3 to 12, preferably acutely angled first combustion air perforations 38 spaced around the side wall 36 and near the base 34 for supplying combustion air to the burn pot 32. The inner housing 30 also defines a substantially horizontal flame conduit 50 connected to and communication with the upper end of the burn pot 32, the flame conduit 50 including a substantially horizontally extending frustoconical venture tube 52 that decreases in diameter in a direction away from the burn pot for creating an constricted venture flow alone the flame conduit 50.

The inner and outer housings 10, 30 are preferably make of steel of sufficient thickness, e.g. ¼ to ½ inches, to withstand the heat and gases of the burning process. The inside diameter of the flame conduit 50 and the contiguous inlet end of the venture tube 52 is about 4 inches, plus or minus 1 inch, and the inside diameter of the outlet end of the venture tube 52 as will as a flame outlet 56 extending horizontally from the flame conduit, is about 1 inch less, or about 3 inches, plus or minus 1 inch. This dimensions, as well as the lengths and diameters of the other parts of the inner and outer housing can be changed and selected for the desired size and heat capacity of the pellet burner, however.

The venture tube 52 has a plurality, e.g. 20 to 40, circumferentially and axially spaced secondary air perforations 54 each, as shown in FIG. 2, at a first acute angle A to an axis of the flame conduit 50, and at a second acute angle B to a circumferential direction around the venture tube 52. Angles A and B are preferably 30 degrees but may be from 10 to 50 degrees. The secondary air perforations 54 communication with the plenum 12 for supplying swirling secondary combustion air to the venture flow that rotates in a selected direction, e.g. clock wise as the flow exits the conduit 50. The flame conduit 50 has its flame outlet 56 extending out through the outer housing outlet 16.

An ignition tube 40 is connected to the inner housing side wall 36 at an off-center location, that is, in axial mis-alignment with the geometric center of the burn pot 32, to define an off-center ignition passage communicating with the burn pot 32. The ignition tube containing at least one second combustion air perforation 42 communicating the plenum 12 with the ignition passage for supplying additional combustion air to the burn pot in an off-center direction into the burn pot for swirling second combustion air from the ignition tube 40 in the burn pot that rotates in the same selected direction (i.e. clock wise but this time in an upward flow direction) as the swirling secondary combustion air in the venture flow in venture tube 52.

Ignitor means 44, for example a heating cartridge of known design, in the ignition passage 40 raises the temperature of air in the burn pot to the auto-ignition temperature of fuel pellets in the burn pot, e.g. 500 to 900 degrees F., and the arrangements is supplied with fuel by pellet supply means 60 connected to at least one of the inner and outer housings 10 and/or 30, for supplying fuel pellets, e.g. standard wood pellets, to the burn pot 32. With activation of the blower 20 and the ignition means 44, and upon reaching the auto-ignition temperature of fuel pellets in the burn pot 32, and further with first and second combustion air from the first and second combustion air perforation 38 and 42 establishing a swirling flame in the burn pot that travels to the flame conduit 50 and is further swirled by the swirling secondary combustion air from the secondary air perforations 54, the venture flow shaping the flame, a clean, hot, symmetrical flame 300 exits from the flame outlet 56 as shown in FIG. 1. Control is provide by a control box schematically shown at 200 in FIG. 1 that is electrically connected to motors of the blower 20 and the pellet feeder 60, as well as to the ignitor 44.

As illustrated in FIGS. 2 and 3, burn pot 32 may have a substantially circular horizontal cross section, which includes either a proper circle or an oval or other curved, closed loop shape, the first combustion air perforations 38 being at an acute angle of 10 to 50 or preferably 30 degrees to a radial direction of the cross section for further enhancing the swirling of the combustion air in the burn pot in the same selected direction as the swirling effect of the perforations 54 in the venture tube 52, e.g. clock wise as the flames, gases and ash rise from the burn pot 32.

The arrangement also advantageously includes a central flow directing cone 70 on the base 34 and in the burn pot, for channeling and further swirling combustion air in the burn pot around the cone 70. Rather than a simple cone in the burn pot with its substantially circular horizontal cross section and its angled first combustion air perforations 38, the cone advantageously comprises a flow directing cone assemble 70 on the base 34 for channeling and further swirling combustion air in the burn pot around the cone assembly 70, the cone assembly having a lower cylindrical platform 72 fixed to the base 34 and a conical top 74 tapering upwardly from the platform 72.

The burn pot 32 preferably has a substantially cylindrical side wall 36 with a circular horizontal cross section, but as shown in FIG. 5, the side wall 36 may be frustoconical with a broader lower end at the base 34 and a circular horizontal cross section. Alternatively the burn pot, as shown in FIG. 4 may have a substantially square or rectangular side wall 36, the first combustion air perforations 38 being straight or, advantageous at an acute angle to sides of the polygonal side walls for further enhancing the swirling of the combustion air in the burn pot. The polygonal burn pot 32 can be used with or without the cone 70, although the cone helps better form the initial flame.

In FIG. 6 the arrangement includes a burner adaptor 80 for firing a boiler or furnace 100 with pellets or at least one other fuel, the burner adaptor having two input walls 82 and 84 that are at an obtuse angle to each other (e.g. 120, or about 100 to 160 degrees), each wall having a flame receiving opening 86, the flame outlet 56 of the flame conduit 50 of the inventive burner with outer housing 10, being fixed to one of the input walls 82 and around the flame receiving opening 86 of the one input wall. Another burner 120 for burning at least one other fuel like oil or natural gas of propane, has a flame conduit fixed to the other input wall 84 around the flame receiving opening 86 of the other input wall. An adaptor enclosure 88 is connected the two the input walls 82 and 84 and connects them to the boiler or furnace 100, for communicating the flame receiving openings 86 of the two input walls 82 and 84 with a combustion chamber 110 of the boiler or furnace 100. A closure plate 90 is pivotally mounted to the adaptor enclosure 88 for pivoting between first and second positions for alternatively closing the flame receiving opening 86 of one input wall at a time while being spaced from the flame receiving opening 86 of the other input wall, so that only flame from only one of the flame receiving opening 86 will enter the combustion chamber 110. Safety switches (not shown) are used to prevent the burner with closed opening 86 from being fired and the obtuse angle between walls 82 and 84 allows either burner to efficiently fire the boiler/furnace 100 without having to install only one or the other burner to the boiler, since the flame axis from both burners will be at an acute angle to the boiler's combustion chamber axis.

The pellet supply means 60 is of known design but must include a down pipe 62 connected either at an acute angle to the burn pot side wall 36 as shown in FIGS. 3 and 5, for example, or has a substantially vertical down pipe 62 connected to a top of the inner housing 30 near the flame conduit 50 at a location that is vertically aligned with the burn pot 32 as shown in FIGS. 1, 4, and 8 for example. With reference to FIG. 1, the pellet supply means 60 with substantially vertical down pipe 62 connected to the inner housing for supplying pellets to the burn pot 32, further includes an auger pipe 64 containing an auger 66 for conveying pellets and connected to the down pipe 62, a hopper 68 connected to the auger pipe 64 for storing a supply of pellets and an auger motor 69 connected to the auger for rotating the auger to supply pellets from the hopper to the down pipe.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles. 

1. An arrangement for burning pellets to fire a combustion chamber of a boiler or furnace, the arrangement comprising: an outer housing (10) defining a first air space with an air inlet (14) for air and an outer housing outlet (16); an air blower (20) connected to the housing (10) for supplying air to the air inlet (14) and into the first air space for pressurizing the first space; an inner housing (30) mounted in the outer housing (10) with the first air space surrounding at least some of the inner housing for defining an air plenum (12) between the inner and outer housings, said inner housing (30) defining a vertically extending burn pot (32) having an upper end and a lower base (34) with a side wall (36) extending around and vertically from the base (34), the side wall (36) including a plurality of first combustion air perforations (38) spaced around the side wall (36) and near the base (34) for supplying combustion air to the burn pot (32), the inner housing (30) defining a substantially horizontal flame conduit (50) connected to and communication with the upper end of the burn pot (32), the flame conduit (50) including an substantially horizontally extending frustoconical venture tube (52) that decreases in diameter in a direction away from the burn pot for creating an constricted venture flow alone the flame conduit (50), the venture tube (52) having a plurality of circumferentially and axially spaced secondary air perforations (54) each at a first acute angle (A) to an axis of the flame conduit and at a second acute angle (B) to a circumferential direction around the venture tube (52), the secondary air perforations communication with the plenum (12) for supplying swirling secondary combustion air to the venture flow that rotates in a selected direction, the flame conduit (50) having a flame outlet (56) extending out through the outer housing outlet (16); an ignition tube (40) connected to the inner housing side wall (36) at an off-center location and defining an off-center ignition passage communicating with the burn pot (32), the ignition tube containing at least one second combustion air perforation (42) communicating the plenum (12) with the ignition passage for supplying additional combustion air to the burn pot in an off-center direction into the burn pot for swirling second combustion air from the ignition tube (40) in the burn pot that rotates in the same selected direction as the swirling secondary combustion air in the venture flow; ignitor means (44) in the ignition passage for raising the temperature of air in the burn pot to an auto-ignition temperature of fuel pellets in the burn pot; and pellet supply means (60) connected to at least one of the inner and outer housings (10, 30) for supplying fuel pellets to the burn pot, so that, with activation of the blower (20) and the ignition means (44) and upon auto-ignition of fuel pellets in the burn pot, first and second combustion air from the first and second combustion air perforation (38, 42) establishes a swirling flame in the burn pot that travels to the flame conduit (50) and is further swirled by the swirling secondary combustion air from the secondary air perforations (54), the venture flow shaping the flame into a clean, hot, symmetrical flame that exits from the flame outlet (56).
 2. The arrangement according to claim 1, wherein the burn pot has a substantially circular horizontal cross section, the first combustion air perforations (38) being at an acute angle to a radial direction of the cross section for further enhancing the swirling of the combustion air in the burn pot, the arrangement including a flow directing cone (70) on the base (34) for channeling and further swirling combustion air in the burn pot around the cone (70).
 3. The arrangement according to claim 1, wherein the burn pot has a substantially circular horizontal cross section, the first combustion air perforations (38) being at an acute angle to a radial direction of the cross section for further enhancing the swirling of the combustion air in the burn pot, the arrangement including a flow directing cone assemble (70) on the base (34) for channeling and further swirling combustion air in the burn pot around the cone assembly (70), the cone assembly having a lower cylindrical platform (72) fixed to the base (34) and a conical top (74) tapering upwardly from the platform.
 4. The arrangement according to claim 1, wherein the burn pot has a substantially cylindrical side wall (36) with a circular horizontal cross section, the first combustion air perforations (38) being at an acute angle to a radial direction of the cross section for further enhancing the swirling of the combustion air in the burn pot, the arrangement including a flow directing cone (70) on the base (34) for channeling and further swirling combustion air in the burn pot around the cone (70).
 5. The arrangement according to claim 1, wherein the burn pot has a frustoconical side wall (36) with broader lower end at the base (34) and a circular horizontal cross section, the first combustion air perforations (38) being at an acute angle to a radial direction of the cross section for further enhancing the swirling of the combustion air in the burn pot, the arrangement including a flow directing cone (70) on the base (34) for channeling and further swirling combustion air in the burn pot around the cone (70).
 6. The arrangement according to claim 1, wherein the burn pot has a substantially rectangular side wall (36), the first combustion air perforations (38) being at an acute angle to sides of the rectangular side wall for further enhancing the swirling of the combustion air in the burn pot.
 7. The arrangement according to claim 1, including a burner adaptor (80) for firing a boiler or furnace (100) with pellets or at least one other fuel, the burner adaptor having two input walls (82, 84) that are at an obtuse angle to each other, each wall having a flame receiving opening (86), the flame outlet (56) of the flame conduit (50) being fixed to one of the input walls around the flame receiving opening of the one input wall, another burner (120) of at least one other fuel having a flame conduit fixed to the other input wall around the flame receiving opening of the other input wall, an adaptor enclosure (88) connecting the two input walls (82, 84) to a boiler or furnace (100), for communicating the flame receiving openings (86) the two input walls (82, 84) with a combustion chamber (110) of the boiler or furnace (100), and a closure plate (90) pivotally mounted to the adaptor enclosure (88) for pivoting between a first and second positions for alternatively closing the flame receiving opening (86) one input wall at a time while being spaced from the flame receiving opening (86) of the other input wall so that only flame of one of the flame receiving opening (86) will enter the combustion chamber (110).
 8. The arrangement according to claim 1, wherein the pellet supply means (60) includes a down pipe (62) connected at an acute angle to the burn pot side wall (36).
 9. The arrangement according to claim 1, wherein the pellet supply means (60) includes a substantially vertical down pipe (62) connected to a top of the flame conduit (50) at a location that is vertically aligned with the burn pot (32).
 10. The arrangement according to claim 1, wherein the pellet supply means (60) includes a substantially vertical down pipe (62) connected to the inner housing for supplying pellets to the burn pot (32), an auger pipe (64) containing an auger (66) for conveying pellets and connected to the down pipe (62), a hopper (68) connected to the auger pipe (64) for storing a supply of pellets and an auger motor (69) connected to the auger for rotating the auger to supply pellets from the hopper to the down pipe. 